Seepage through an earth mass refers to the flow of water or other fluids through the soil or rock. It occurs when there is a difference in hydraulic head or water pressure between two points in the soil. Seepage can have significant effects on the stability and performance of engineering structures such as dams, retaining walls, and foundations.

The direction of seepage is an important factor to consider in the analysis and design of these structures. It is determined by the distribution of hydraulic head or pressure within the soil mass. The direction of seepage can be understood by considering the equipotential lines and streamlines.

1. Equipotential lines:
Equipotential lines are imaginary lines that connect points in the soil mass that have the same hydraulic head or pressure. They represent surfaces of equal potential energy. In the context of seepage, these lines are perpendicular to the flow direction.

2. Streamlines:
Streamlines, on the other hand, are imaginary lines that represent the path followed by a fluid particle as it flows through the soil. They are tangent to the velocity vector at each point. In the context of seepage, these lines are parallel to the flow direction.

Seepage occurs perpendicular to equipotential lines:
When seepage occurs through an earth mass, it follows a path perpendicular to the equipotential lines. This is because seepage is driven by the difference in hydraulic head between two points, and the flow always occurs from a higher head to a lower head. The flow is perpendicular to the equipotential lines because the equipotential lines represent surfaces of equal head.

Therefore, the correct answer is option 'C' - seepage occurs perpendicular to the equipotential lines. This understanding is crucial in the analysis and design of engineering structures to ensure proper drainage and prevent potential issues caused by seepage, such as soil erosion and instability.